The present invention relates to a thermal transfer printer that will transfer an imaging compound, such as a wax, wax resin or wax resin composite, or a dye from a carrier ribbon, film or web to a substrate, as shown, a disc shaped substrate such as a compact disc (CD) or digital versatile disc (DVD) using linear printing across transverse (chordal) lines on the circular disc, which vary in length as the disc progresses underneath the thermal printhead. The velocity of movement of the compact disc is varied along with a platen position and engagement force to efficiently use available power so as to compensate for the changing power required by the printhead for printing differing print line lengths and also space the printhead from raised rims around the central opening of DVD""S.
Thermal printing technology for CDs and DVDs and recordable compact discs (CD-Rs) incorporate pivotally mounted heads and linear platens with resilient surfaces and expensive clamping mechanisms for the disc. Loading thermal ink ribbons is cumbersome, inconvenient and time-consuming because the ribbon must be threaded through the rollers and the printhead and manually re-attached to a take-up core.
Current technology for printing onto CDs utilizes expensive head actuating and force modifying mechanisms. The printhead is moved on pivotally mounted arms that extend substantially beyond the envelope of the printhead, with a linearly driven carriage that has to hold the disc over a flat resilient surface with a clamping device that moves with the carriage. Threading the ribbon through the printhead and mounting ports of the presently available printers is a tedious job which includes taping the ribbon to the carriage, then taping the ribbon after the carriage is driven into the printer. This leads to large, high-cost thermal transfer CD, CD-R and DVD printers. It is desirable to substantially reduce the size in order to take less space for the CD printers, as well as manufacturing costs and user interaction.
The invention described herein is a thermal transfer printer utilizing numerous cost-saving and space saving design features that will improve the technology and give the printer the potential to be used in a typical computer drive bay.
Conventional techniques for thermal printing onto circular objects, such as a CD disc, cause the circular object to be printed to pass beneath a thermally activated printhead at a uniform rate of speed. The present invention varies the speed at which the circular object passes beneath the printhead to manage the power needed for the printing process and varies the force on and position of the disc as a function of the position of the disc under the printhead. This permits the printer to utilize a non-dedicated and limited power source, such as a personal computer power supply and also to reliably print on DVDs which have a raised rim around the center portion of the disc.
The present invention relates to a thermal transfer printer utilizing a printhead cartridge that mounts into a frame and prints on CDs, CD-Rs and DVDs reliably. The printhead is mounted so that when it is in place on the printer frame, it does not move relative to the frame. A platen roller is mounted so that it and a carrier for the disc can be moved against the printhead under a spring force that is controlled externally. The force is preferably varied as a function of disc position, taking into account the length of the print line being printed, also, the platen can be moved to cause the disc to be spaced from the printhead to clear irregularities on the disc surface, such as the raised rim ring that is present at the center portion of the DVDs.
In one form shown, a schematic representation is made for a spring loading of the platen, and also in a more detailed form, the platen is mounted onto a pivoting frame having side arms that are supported on a pivot. The frame mounts not only the platen, but also additional drive rollers for the substrate or disc carrier. The force with which the platen, carrier and substrate are urged against the printhead during the printing operation can be selected to be at a desired level, and can be changed by a cam that acts on a cam follower connected to the platen frame by springs so that the frame pivots toward the printhead under spring load. The platen is resilient to provide some cushioning load of the cartridge and the substrate, (CD or CD-R for example) against the printhead, which is held in a fixed position. The force applied by the spring to the platen is changed by changing the position of the cam. This cam position is correlated to the position of the disc under the platen, so the force can be varied as a function of print length. Also, if the substrate has irregularities (whether a disc or a rectilinear card) the platen can be dropped so the printhead clears the irregularities and then returns to printing after the irregularities or raised portion passes by.
The drive motor for the platen and any other drive motors used to move the substrate or disc is varied in speed, to vary the velocity of movement of the disc as a function of the disc position relative to the print line of the printhead. The speed change is relative to the length of the line actually being printed at any position of the disc. Alternatively, the speed can change relative to the length of the potential print line, even if the entire line is not printed upon. The change in speed can be correlated with a change in cam position for changing the force on the printhead at the same time and dropping the platen and substrate for clearing a DVD raised rim or other known irregularity.
The power required by the thermal printhead is proportional to the number of heat resistors energized on the length of the print line. When the print line is shorter, more power is available to the disc drive system which, as a result, can be driven faster. A stepper motor drives the disc and its support past the printhead at a speed which varies as the printed substrate (typically a CD, DVD or CD-R) moves past the printhead. By varying the speed of the substrate, an elegant solution is provided to the problem of maximizing print speed within the constraints of the limited power supply within a personal computer. Since the invention is engineered to fit within a standard slot within a personal computer, or other devices with limited power it must share the power supply provided by the personal computer with the other functions within the personal computer requiring power. The most efficient methodology to provide high print speed and low demand on the power is to move the disc and its carrier as quickly as is possible without impacting print quality. When the disc carrier is originally moved into contact with the printhead, the print line is short. The print line length increases until a position near the diameter of the substrate is reached (the disc has a center hole) then the length of the print line decreases until it reaches a local minimum at the center. The second half of the disc is symmetrical with respect to the center. The speed varies inversely to the print line length. After starting at 0, the speed quickly reaches a maximum near the edge of the disc. The speed is then reduced to a minimum at the point where the print line is longest. Then the speed is increased until it reaches a local maximum in the center. There is compensation for the presence of a center hole which is standard for holding such discs and the force from the platen can be reduced in the center portion. The platen can be lowered in center portions to clear the rim on a DVD, and speed increased as that portion of a DVD moves under the printhead.